FIG. 1 shows transmission of a signal from a sender to a receiver in a telecommunication system. The information to be transmitted is conveyed over a transmission channel, such as a radio channel, modulated into a form suitable for the channel. Known methods of modulation include amplitude modulation, where the information is contained in the signal amplitude, frequency modulation, where the information is included in the signal frequency, and phase modulation, where the information is included in the signal phase. Non-ideal features of the transmission channel, such as signal reflections, noise and interference caused by other connections, cause changes in the signal containing the information, which is why the signal perceived by the receiver is never an exact copy of the signal sent by the sender. Information to be sent in digital systems can be made to better withstand non-ideal features of the transmission path with the aid of channel coding. At the receiving end the receiver will correct the received signal with a channel corrector based on channel characteristics which it knows and it will undo the modulation used on the transmission channel as well as the channel coding.
Besides attenuation of the amplitude, a sent signal will broaden on the transmission channel both at frequency level and time level. The information included in the signal by some modulation method will hereby also change. Broadening of the signal is caused especially in radio systems mainly by multipath propagation, which is shown in FIG. 2. In the figure a signal is examined which travels from the base transceiver station BTS to a mobile station MS in a mobile station system. The signal travels from the base transceiver station along a straight route, the length of which is Lstraight. In addition, the mobile station perceives two beams, which are reflected from an obstacle and the route lengths of which are Lrefl1 and Lrefl2 respectively. The mobile station receives the signal conveyed by the reflected beam 1 after a delay xcex94T1=(Lrefl1xe2x88x92Lstraight) c and the signal conveyed by beam 2 after a delay xcex94T2=(Lrefl2xe2x88x92Lstraigh)/c later than the signal which propagated straight (c=speed of light). Thus, the receiver perceives the sent signal as three signals arriving at slightly different times and from different directions and summing up as one, which causes overlapping of symbols sent in succession, that is, Inter-Symbol Interference ISI.
Besides multipath propagation, inter-symbol interference is caused by the modulation methods used. E.g. in a Gaussian Minimum Shift Keying method (GMSK) used in a GSM system, changes between successive signals are smoothed to save the frequency band of the radio channel in such a way that the effect of an individual symbol will extend over the time of three symbol periods. Since the effect is on the signal phase, it will cause a non-linear component in the inter-symbol interference. The GMSK method is described more closely e.g. in the GSM 05.04 standard published by the ETSI (ETSI=European Telecommunications Standards Institute).
In order to correct changes caused by the channel, there must be sufficiently accurate knowledge of channel characteristics at the receiving end. Known channel estimation methods are the use of a Training Period TP and blind channel estimation. In blind channel estimation, an estimate of channel characteristics is maintained by defining from the received signal the statistically most likely transmitted signal. If the signal reconstructed from the received signal with the aid of estimated channel characteristics is not probable or even possible, the estimate of channel characteristics is changed.
In channel estimation methods using a training period, the idea is to include such a training period in the transmitted signal, the contents of which are known to the receiver. By comparing the received and distorted training period, which has travelled through the channel, with the training period which it knows and which was sent to the channel, the receiver will obtain information on channel characteristics. Based on the information obtained the receiver may correct any distortions caused by the channel also from such other transmitted information conveyed in other parts of the burst which the receiver does not know beforehand.
FIG. 3 shows how a training period is located in a burst for use in digital radio communication. In the figure the training period is located in the middle part of the burst, whereby the average distance of information bits from the training period is minimised. A first half-burst containing information to be transmitted is located before the training period, and a second half-burst containing information is located after the training period. In addition, at the ends of the burst there are also tails needed for perceiving the ends of the burst and a safety time used for preventing overlapping of successive bursts.
FIG. 4 shows the occurrence of interference caused to one another by simultaneous connections. In the figure, three mobile stations MS1, MS2 and MS3 communicate with base transceiver stations BTS1, BTS2 and BTS3. The signal received by base transceiver station BTS1 contains a signal S1 sent by mobile station MS1 and shown by a solid line, the strength of which depends on the transmission power used by mobile station MS1, on fading on the radio path between mobile station MS1 and base transceiver station BTS1 and on the antenna""s sensitivity in the direction of arrival of the beam. Typically, radio path fading is smaller the closer the mobile station is located to the base transceiver station. Besides signal S1, the signal received by the base transceiver station contains signal components I21 and I31 resulting from signals sent by mobile stations MS2 and MS3. The receiver perceives signals S1, I21 and I31 as a straight beam but also as several reflections coming from different directions, which are not however shown in the figure for the sake of simplicity. Components I21 and I31 will cause interference in the reception, unless they can be filtered away from the signal received from the base transceiver station. Correspondingly, the signal sent by mobile station MS1 causes in the signals received by base transceiver stations BTS2 and BTS3 signal components I12 and I13 which may cause interference in receptions. Components of a similar kind will also occur in the signals received by the mobile stations from the base transceiver stations.
If signal components I21 and I31 are on the same channel as signal S1, they can not be removed by filtering. Also signals which are on some other channels than the same channel may cause interference. Since e.g. in systems using FDM frequency division such channels which are beside each other at the frequency level are always slightly overlapping due to an optimally efficient use of the frequency spectrum, interference will also be caused in the reception by signals on the adjacent channel. Similarly, when using CDM code division, connections using codes which resemble each other too much will cause interference to each other. However, so-called adjacent channel interference caused by signals on other channels are considerably smaller than the interference caused by equally powerful signals on the same channel.
Thus the magnitude of interference caused by connections to one another depends on the channels used by the connections, on the geographical location of the connections and on the transmission power used. These may be affected by such systematic channel allocation to different cells which takes interference into account, by transmission power control and by averaging of the interference experienced by the different connections.
Besides by the methods mentioned above, connection interference can be reduced by making use of the fact that the desired signal and the interfering signal typically arrive at the receiver from different directions. The interference can hereby be reduced by directing the antenna adaptively so that its sensitivity is greatest in the direction of the desired beam and considerably smaller in the direction of arrival of interfering beams. The antenna is directed by using several antenna elements, the signal phase of which is controlled. This method is called Spatial Division Multiple Access method (SDMA). Using the SDMA method signals can be distinguished not only by their frequency and time slot channel but also by their direction of arrival. Thanks to this the same channel may be used in the method several times even inside one and the same cell.
FIG. 5 shows the basic principle of a SDMA system. The base transceiver station perceives signals S1 and S2 sent by two mobile subscribers MS1 and MS2 with several different antenna elements A1 . . . A4. The method is based on the fact that although the signal x1 . . . x4 received by each individual antenna element is a combination of two separate transmitted signals S1 and S2, different antennas will perceive different combinations. Under these circumstances the sum signals x1 . . . x4 received by the antennas form signals S1 and S2 that can be distinguished from each other thanks to the different training periods used by the mobile subscribers.
FIGS. 6 and 7 show known methods of merging signals obtained from an antenna vector. In the arrangement shown in FIG. 6, signals x1 . . . x8 of antennas A1 . . . A8 are fed directly to an optimum merger, such as a MuliDimensional Maximum Likelihood Sequence Estimator MD-MLSE. The MD-MLSE may be implemented e.g. with a vectored Viterbi algorithm. To the merger algorithm is supplied, besides the input vectors x1 . . . x8, a channel characteristic estimator H with the aid of which inter-symbol interference is reduced. In addition, the estimator may supply to the combination algorithm MD-MLSE information Q on any correlation between different signals.
It is a problem with direct optimum merging that the complexity of merging algorithms will typically increase exponentially in relation to the input signals. Hereby in bigger systems using e.g. eight antenna elements implementation of the algorithm demands very high computation power. Another problem with this method is its relatively high sensitivity to noise.
FIG. 7 shows another known method of interpreting a signal received by an antenna vector. The method is presented in the publication S. Ratnavel et al., xe2x80x9cMMSE Space-Time Equalization for GSM Cellular Systemsxe2x80x9d, Proceedings of the IEEE Vehicular Technology Conference, Atlanta, USA, 1996. The method separates from each other the linear inter-symbol interference caused by the radio channel and the non-linear inter-symbol interference caused by the GMSK modulation used in GSM. The data to be transmitted and the training period are first separated from each other from the signal x1 . . . x4 of the antenna vector A1 . . . A4. The characteristics of the radio channel are estimated with the MMSE method (Minimum Mean Square Error)by comparing the received training periods with the modulated training period. The MMSE method minimises the square sum of the deviation between the received training period corrected by the channel corrector and the true training period. Information obtained simultaneously from all antenna elements is used in the definition of channel characteristics. The estimation results in coefficients Wi which are used in the time and place dependent channel corrector and the number of which is Mp, wherein M is the number of antennas and p is the number of potential values of delay differences of signals which are taken into account in the channel correction. In a GSM system, four bit periods, which is equal to approximately 15 microseconds, are used as the time broadening taken into account by channel correctors.
Since the correction coefficients Wi used with different antenna signals are calculated using information on the radio channel between transmitter and elements which is available simultaneously from all antenna elements, the coefficients also contain phase information between the antennas. With the aid of this information the receiving beam of the antenna is directed towards the transmitter sending the desired signal.
The time and place dependent channel corrector corrects the signal obtained from antenna A1 with coefficients w10 . . . w14, and the signal obtained from antenna A2 with coefficients w20 . . . w24, etc. The channel corrected signals of the antennas are summed together, and the resulting corrected signal is supplied to the GMSK demodulator, which will undo the modulation used for the signal on the radio path.
The finite length of the used training period is a problem with this method. If the training period length is e.g. 26 bits, such as e.g. in a GSM system, and channel correction is used for correcting time broadening, the length of which is four bit periods, then 22 bits are available for use in the estimation of channel characteristics. Based on these 22 bits it is possible unambiguously to define no more than 22 parameters, so the result Mp of the number of antennas M and the signal""s time broadening p taken into account by the channel correction must be less than 22. Due to the limitations caused by this over parametrisation of the estimator the number of antennas is limited even theoretically to four, which again reduces the receiver""s interference and noise tolerance.
Thus, the problems with state-of-the-art systems are high complexity and a relatively high sensitivity to noise or a limitation of the number of antennas which can be used. It is an objective of the present invention to eliminate or at least to aleviate these state-of-the-art problems. This objective is achieved with the methods and equipment presented in the independent claims.
The inventive idea is to perform interpretation of the received signal in two steps. In the first step, dynamically directed signal beams are formed of a multidimensional signal obtained from an antenna vector including several antenna elements. In the second step, the directed signals are supplied to an optimum merger, which concludes the transmitted signal from several signal branches used as input.
In a first advantageous embodiment, signals obtained from antenna elements are directed by multiplying the signal vector obtained from the antenna vector by complex coefficients, which are obtained from an analysis of the direction of arrival of the signal. Since the signal""s direction of arrival changes slowly compared with changes in radio path fading, information obtained over a longer time can be used in the estimation of the direction of arrival, whereby the antenna beam can be directed more accurately. For the signal beams which are obtained from the beam formation and which have experienced different radio channels, estimates of channel characteristics are defined, which are input to the optimum merger together with the beam signals. The number of directed beams is preferably smaller than the number of antenna elements, whereby the complexity of the optimum merger is essentially reduced. The optimum merger deduces the most likely transmitted signal from information obtained from different signal beams.
In another advantageous embodiment, the signals obtained from antenna elements are first divided into smaller groups. The signals of each group are input to their own MMSE estimator which corrects the channel and phases the elements. Each MMSE estimator produces one channel corrected and directed signal, which is switched to the optimum merger. The optimum merger deduces the most likely transmitted signal from signals obtained from different MMSE estimators and containing the same information.